1. Field of the Invention
This invention relates to an oil pump, and more particularly to an oil pump used for vehicle engines.
2. Description of the Related Art
In a conventional oil pump which is used for an internal combustion engine for vehicles, an internal-external gear pump is mainly utilized. A dual type vane pump is conventionally proposed for a power steering device for vehicles as shown in Japanese Patent Publication No. 62 (1987) - 26300. The dual type vane pump is shown in FIGS. 3 and 4, and includes a front pump 1 and a rear pump 2 which are rotatably driven by a common pump shaft 3.
The pump shaft 3 penetrates the front pump 1 and a drive pulley 4 is attached to a projecting end of the pump shaft 3. The front pump 1 is comprised with a front body and a cam ring 6 within which is inserted a rotor 7. A common center plate 8 is common to the front pump and the rear pump 2 comprised of a rear body 9 and a cam ring 11 within which inserted a rotor 10. The front and rear pumps 1 and 2 are integrally connected by means of a plurality of bolts 12 which extend parallel with the pump shaft 3.
The cam ring 6 is fixedly clamped between the front body 5 and the center plate 8 in the front pump 1. The distal ends of a plurality of vanes 13 radially arranged in the rotor 7 contact with an inner circumferential cam surface of the cam ring 6, thereby forming a pump chamber 14 between each pair of vanes 13. An intake port 15 is provided on a contacting surface 8A of the canter plate 8 so as to be located at a circumferential region where the volume of the pump chamber 14 is enlarged in accordance with the rotation of the rotor 7. Further, a discharge port 16 is provided on a contacting surface 5A of the front body 5 so as to be located at a circumferential region where the pump chamber 14 volume is at a minimum. Accordingly, actuating oil sucked from the intake port 15 into the pump chambers 14 is pumped in accordance with the rotation of the rotor 7.
An intake port 24 for the common actuating oil, common to both pumps, is attached to the front body 5 and is communicated with an excess oil passage 31 of a flow control valve 30 interposed within the front body 5. The intake port 24 is further communicated with an inlet 27A opened to the contacting surface 8A of the center plate 8 via a connecting passage 26 formed on the cam ring 6 by a passage port 25 opened to the contacting surface 5A, thereby forwarding the actuating oil from the intake port 24 into the intake passage 21. The actuating oil flows from an inlet port 27B opened to the contacting surface 8B of the center plate 8 to the intake passage 21 via an excess oil passage 33 of a flow control valve 32 disposed in the rear body 9 opposedly located via a connecting passage 29 formed on the cam ring 11. The discharge pressure is applied to the slit grooves in which the vanes 13 slide so that the vanes 13 are pressed radially outward and contact with an inner circumferential surface of the pump chamber 14. The clearance of the vanes 13 in the slit grooves must be maintained with high accuracy in order to prevent binding of the vanes. However, when the vane pump is used for pumping a lubrication oil of the vehicle engine, foreign matter within the oil pan is invaded into the slit groove and the vane pump cannot be operated reliably.